1. FIELD OF THE INVENTION
The present invention relates to the conversion of polymers, particularly the conversion of polymer waste, into industrially useful products. More particularly, it relates to the cracking of polymers mixtures into lower molecular weight products and into light hydrocarbons which can be recycled or used as raw material.
2. DESCRIPTION OF THE PRIOR ART
The polymers are being produced in increasing amounts and most of them are used in the one-use packaging area.
However, the weak tendency of polymers for natural decomposition and the reduction of the number of available rubbish disposal sites lead to search for new directions to treat polymeric material packaging waste.
The incineration is a technique currently used for the treatment of domestic waste. Although it gives good results with polyolefins, due to their complete combustion which gives water and carbon dioxide, this technique is not suitable for other polymers which do not incinerate as well and tend to carbonize and to give off noxious vapors.
The recycling of polymers, as such, only applies for limited applications. Further, it is necessary, after collection, to make a prior sort depending on their nature.
There is thus a need for a polymer conversion process at large scale, particularly a conversion process including cracking mixed polymers waste into lower molecular weight products and into lighter hydrocarbons which can be recycled or used as raw material, for example for supplying oil refineries and petrochemical steam crackers.
The behavior of polymers at high temperature, if need be in the presence of catalysts or appropriate reactants, has been studied for many years and is well known for the person skilled in the art. Depending on the case, it results into depolymerization (for example, for polystyrene pyrolysis or polyethylene terephthalate methanalysis or hydrolysis) or decomposition (for example, during the catalytic treatment of polyethylene).
U.S. Pat. No. 4,151,216 discloses the catalytic cracking of polypropylene at 425.degree.-475.degree. C. on silica-alumina yielding a liquid fuel from 50.degree. C.
U.S. Pat. No. 4,143,086 discloses a fluidized bed catalytic cracking of amorphous polypropylene in the presence of a hydrocarbon feed yielding an effluent containing propylene.
European patent application 414 439 discloses the conversion of high molecular weight polymers into lower molecular weight product by heating the polymer (possibly dissolved) in contact with a zeolitic acid catalyst.
European patent application 276 081 and U.S. Pat. No. 4,584,421 disclose process for the decomposition of polyolefins into two steps; it consists in a thermal cracking followed by a catalytic cracking of the product coming from the first reaction.
However, these processes do not consider the treatment of different polymers mixtures and the resulting problems. It seems, therefore, very important and necessary at this time to have a simple process comprising a minimum amount of steps to treat polymer mixtures. The Applicant has now perfected an integrated process for the conversion of polymers, particularly suitable for the treatment of polymer waste, into industrial useful products.